U.S. Pat. No. 4,602,505 (corresponding to European Patent Publication No. 112156A), entitled "APPARATUS FOR MEASURING VISCOSITY", and issued June 29, 1986 to the inventors, proposes a viscosity measuring apparatus that uses a tuning fork vibrating member of a new idea instead of the conventional single vibrating member type viscosity measuring apparatus. In the single vibrating member type viscosity measuring apparatus, the reaction due to the vibrations of the vibrating member is induced in the support for supporting the vibrating member, resulting in the change in the amplitude value of the vibrating member, and as a result, it is impossible to obtain a high accuracy of the measurement. On the other hand, in the tuning fork vibration type viscosity measuring apparatus, since the apparatus has a pair of vibrating members which are driven in inverse phase relation to each other, the pair of the vibrating members can be operated in a resonance state, and therefore the reaction applied to the support due to the vibrations is little or can be reduced to a negligible degree, resulting in substantial improvement in the accuracy of the measurement.
Further study seeking a higher accuracy of measurement has been continued and necessity for improvements of the above tuning fork vibration type viscosity measuring apparatus has become apparent. One of the improvements is to measure the temperature of a sample quickly and accurately so as to make clear the relative relation between viscosity and temperature as in the case of other types of viscosity measuring apparatuses. Another improvement is to keep the depth of vibrating members in a sample liquid constant to solve the problem of the change in the measured value of the viscosity due to the difference of the depth of the vibrating members immersed in a sample liquid. Other remaining improvements include, for example, the design of the structure of the vibrating members and the way of supporting the vibrating members, almost of all of the problems have been solved and the new tuning fork vibration type viscosity measuring apparatuses are being used for the viscosity measurement.